Which of these numbers is prime? ${33,\ 35,\ 42,\ 79,\ 91}$
A prime number has exactly two factors: 1 and itself. The factors of 33 are 1, 3, 11, and 33. The factors of 35 are 1, 5, 7, and 35. The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42. The factors of 79 are 1 and 79. The factors of 91 are 1, 7, 13, and 91. Thus, 79 is a prime number.